Motor Types and Appropriate Applications
The
following section gives some idea of the applications that
are particularly appropriate for each motor type, together
with certain applications that are best avoided. It should
be stressed that there is a wide range of applications that
can be equally well met by more than one motor type, and the
choice will tend to be dictated by customer preference,
previous experience or compatibility with existing
equipment.
Cost-conscious applications
will always be worth
attempting with a stepper, as it will generally be hard to
beat the stepper’s price. This is particularly true when the
dynamic requirements are not severe, such as “setting” type
applications like positioning a guillotine back-stop or a
print roller.
High-Torque,
Low-Speed, continuous-duty
applications are also appropriate for step motors. At low
speeds, it is very efficient in terms on torque output
relative to both size and input power. Microstepping can
improve low-speed applications such as metering pump drive
for very accurate flow control.
High-Torque,
Low-Speed, continuous-duty
applications suit the servo motor, and in fact, a step motor
should be avoided in such applications because the
high-speed losses can cause excessive motor heating. A DC
motor can deliver greater continuous shaft power at high
speeds than a stepper motor of the same frame size.
Short, rapid
repetitive moves are the natural
domain of steppers of hybrid servos due to their high torque
at low speeds, good torque-to-inertia ratio and a lack of
communication problems. The brushes of the DC motor can
limit its potential for frequent starts, stops and direction
changes.
Low-friction, mainly
inertial loads can be effectively
handled by the DC servo provided the start/stop duty
requirements are not excessive. This type of load requires a
high ratio of peak to continuous torque and in this respect
the servo motor excels.
Very arduous applications
with a high dynamic duty
cycle or requiring very high speeds may require a brushless
motor. This solution may also be dictated when
maintenance-free operation is necessary.
Low-speed, high-smoothness
applications are
appropriate for microstepping or direct drive servos.
Applications in
hazardous environments or in a
vacuum may not be able to use a brush motor. Either a
stepper or a brushless motor is called for, depending on the
demands of the load. Bear in mind that heat dissipation may
be a problem in a vacuum when the loads are excessive.
Stepper Motor Benefits
In general terms the stepper
system offers the lowest-cost solution while the brushless servo
provides the highest overall performance. When selecting the
correct technology there are often other considerations which
influence the choice, such as compatibility with existing
equipment or customer preference. Some applications could be
equally well served by more than one technology. The performance
of a motor is strongly influenced by the type of drive and
controller used and we can assist with the application to match
key components to ensure the optimum performance and solution.
Lowest-cost solution
A stepper motor will always
offer the lowest cost solution. If a stepper will do the job,
use it.
Rugged and reliable
Steppers are mechanically very
simple and apart from the bearings (in common with servos) there
is nothing to deteriorate or fail.
No maintenance
There are no brushes requiring
periodic checking or replacement.
Industry-standard ranges (NEMA
or metric)
Steppers are produced to
standard flange and shaft sizes so finding a second source is
not a problem.
Few environmental constraints
A stepper may be used in just
about any environment, including in a vacuum.
Inherently failsafe
Any conceivable fault within
the drive prevents motion, since the current must be continually
switched for continuous rotation. A brush motor is
internally-commutated and can run away if continuous current is
applied.
Not easily de-magnetised by
excessive current
Owing to the perpendicular
planes of permanent-magnet and alternating flux paths.
Inherently stable at
standstill
With DC flowing in the
windings the rotor will remain completely stationary. There is
no tendency to jitter around an encoder or resolver position.
This is useful in applications like microscope stages.
Can be stalled indefinitely
without damage
There is no increase in motor
current as a result of a stall or jam, therefore no risk to the
motor or drive and minimum chance of mechanical damage.
High continuous torque in
relation to size
Compared with a servo of the
same size, a stepper can produce greater continuous torque at
low speeds.
Only 4 leads required
This minimises the installed
cost, particularly important in applications where connections
are expensive (e.g. vacuum chambers). |
